1. Technical Field
The invention relates to the field of micro-optics and nano-optics, and more particularly relates to waveguide structures for optical interconnection of planar-integrated photonic systems (chip-chip connections) and the connection of planar integrated photonic systems to glass fibers (fiber-chip connection). The invention furthermore relates to a method and a device for producing the waveguide structures.
The field of integrated optics, in particular the area of silicon photonics, meaning the integration of waveguide-based photonic components on silicon or silicon-on-insulator (SOI) substrates, has been the subject of intensive research and development work for several years. This technology permits transferring mature CMOS (complementary metal oxide semiconductor) processes, developed for mass production of integrated electronic circuits, to the integrated photonic. Nano-photonic systems with high complexity and a plurality of functionalities can thus be integrated into the smallest possible space and produced on an industrial scale. The areas of application for such systems are primarily in the field of data transmission, as well as for optical measuring technology and sensing technology. The economic potential of silicon photonics is high and first products are already commercially available.
However, the design and connection technology for integrated photonic systems has increasingly proven to be an obstacle for further commercialization. Complex photonic systems are based on lateral single-mode planar integrated waveguides. Waveguide structures are called “lateral single mode” for which in each polarization only the basic mode can propagate. Lateral single-mode waveguides thus generally have two guided modes with different polarizations, for example called “quasi TE” and “quasi TM”. A low-loss optical connection of these systems is therefore only possible with waveguide structures which permit an efficient connection to the basic modes of the integrated waveguide. For the optical connection of integrated photonic systems, standard single-mode fibers are generally used which are affixed directly to the chip, using a high number of manual operational steps. This leads to a correspondingly low integration density of optical chip-to-chip connections and results in high packaging costs which, in part, amount to more than 50% of the total costs for the system. In the field of microelectronics, the development of reliable “wire bonding” techniques was a basic requirement for economic success. Methods with a comparable throughput and degree of automation have so far not been available for the field of photonics.
For connecting integrated optical components, passive positioning methods are primarily used for which a transmitter chip as well as a receiver chip must be positioned with corresponding devices and sufficient precision, relative to the waveguide. The precision requirements are for the most part determined by the cross-sectional surfaces of the waveguides. Thus, systems are often preferred for the mass production of systems which are based on multi-modal waveguides with large cross-sectional surfaces. In recent years, techniques have been developed which permit, for example, a multi-modal connection of surface-emitting laser sources (so-called vertical cavity surface emitting lasers (VCSEL)) on a transmitter chip with planar integrated photo detectors on a receiver chip.
However, these methods cannot be used for connecting lateral single-mode integrated optical waveguides. Waveguide diameters in the field of silicon photonics typically are noticeably below 1 μm. The resulting precision requirements cannot be met satisfactorily with adjustment techniques only.
Further, over the last years, several photonic integration platforms have gained maturity, each of them having individual strengths and weaknesses. III-V-compound semiconductors have become the mainstay for optical sources and amplifiers, whereas silicon photonics enables efficient co-integrating of modulators and detectors together with WDM functions, and optical waveguides based on silicon nitride are used for high-performance passive devices. However, there is currently no flexible and cost-effective method that could combine these technologies into a joint module.
Single-mode integrated optical components are presently connected almost exclusively with the aid of single-mode fibers which are connected directly to the end face of an integrated waveguide. In the process, the fibers and the waveguide facets must be positioned and attached with high precision, relative to each other. As a rule, this is achieved by measuring the optical coupling efficiency during the alignment and by maximizing the positioning of the fiber tip, wherein this is also referred to as “active alignment.” The mode field adaptation between the single-mode fiber and the integrated waveguide as a rule occurs by giving the fiber tip a special form which ensures a focusing of the exiting light (so-called “lensed fibers”). In addition, integrated optical tapers are often used which optimize the coupling efficiency between the focused mode field for the single-mode fiber and the integrated waveguide.
This method was developed for individual fiber-chip connections which are not scalable for a large number of optical connections and can therefore no longer meet the requirements of a high-quality integrated photonic system. The geometric dimensions of standard single-mode fibers (diameter approximately 125 μm), for example, limit the achievable integration density. In addition, the high expenditure for an active adjustment is no longer acceptable for the mass production of integrated optical systems. The active adjustment with passive integrated optical systems is furthermore difficult since these systems do not contain an inherent light source which would allow detecting and optimizing the coupling efficiency during the adjustment operation. Passive adjustment methods continue to suffer from poor reproducibility of the coupling efficiency. Furthermore problematic are the generally high optical losses with chip-fiber couplings (50% are no rarity) which, above all, can be traced back to poor mode field adaptations between the single-mode fiber and the integrated waveguides. Since the integrated waveguides generally are planar structures, a mode field adaptation frequently is possible only in the substrate plane.
2. Prior Art
German patent document DE 10 2007 055530 A1 describes a method for laser-beam processing of a work piece. Among other things, the specification discloses a method for finding planar surfaces within the work piece to be processed. Three-dimensional structures cannot be detected with this method and the production of waveguides as well as their connection to pre-positioned structures is not mentioned therein.
German patent document DE 601 14 820 T2 describes use of multi-photon induced photo structuring methods for producing three-dimensional optically functional structures in polymer or oligomer materials. The description is concentrated on the lithography methods and the resist materials. The adaptation and connecting of the generated structures to pre-positioned components is not mentioned.
German patent document DE 601 30 531 T2 describes production of optical waveguides of a light-hardening resin. The description is focused on the sequences of different illumination steps, which are used to define the core region and the sheath region of the waveguide. The resins here are structured along the propagation direction for a light beam. The waveguides thus consist of a series of, and in some parts straight, sections. Free form curves with variable waveguide cross sections cannot be created with this method. Accordingly, the problem definition upon which the present invention is based cannot be solved this way.
German patent document DE 10 2007 038 642 A1 describes three-dimensional structuring of waveguides with variable cross-sectional geometries in propagation direction by using multi-photon processes. A local increase of the refractive index, induced by the radiation, is used for the light guidance. The consequently achievable index contrast is low (typically 0.005), so that realization of compact photonic wire bonds is not possible with this method. The adaptation and/or the connection of the waveguide structures, generated in this way, to pre-positioned optical components is not the subject matter of the specification.
International patent application publication WO 2009/021256 A1 describes a method for producing optical waveguides on polymer substrates. The specification concentrates on the lithographic method and the resist materials, wherein the three-dimensional structuring with the aid of two-photon absorption processes is also mentioned. The adaptation and/or the connection to the waveguide structures, generated in this way, to pre-positioned optical components is not part of the subject matter of the specification.
European patent document EP 0 689 067 A2 describes a method for the optical structuring of connecting waveguides between pre-positioned components. The light beam used for the structuring is radiated directly from the ends of the waveguides to be connected into a non-linear optical material. In regions of high optical intensity, meaning along the light rays and in particular at the crossing points of light rays, an optically induced polymerization reaction takes place, which leads to forming waveguide structures along the paths for radiating in light. These waveguide structures are oriented per definition on the optical elements to be connected, but the waveguide geometries that can be created with this method are strongly limited. In particular, it is not possible to generate pre-computed and optimized free-form waveguides. The achievable index contrast is furthermore very low and the components to be connected must be positioned, relative to each other, with high accuracy. The use of passive components is furthermore also made more difficult in that these are frequently not transparent for single-photon processes at the lithographic wavelength. The use of multi-photon polymerization processes in most cases fails because the required capacities frequently cannot be transported in passive structures. The problem definition upon which the present invention is based cannot be solved with this method.
German patent document DE 19545 721 C 2 describes a method for producing optical micro-components on fiber end surfaces and/or laser facet. For this, the position of the region where the optical micro-component is to be generated is first detected with an imaging method. Based on the data obtained, the optical micro-component is then generated with high relative accuracy on the fiber end surface or the laser facet. The term optical micro-component in this case refers to lenses or prisms. The connection and geometric adaptation of waveguides on pre-positioned optical components is not mentioned and is not feasible with this method since the described imaging method does not permit the three-dimensional position detection. The described method thus cannot be used to solve the problem defined for the present invention.
Schmid, G.; Leeb, W.; Langer G.; Schmidt, V & Houbertz, R., “Gbit/s transmission via two-photon absorption-inscribed optical waveguides on printed circuit boards;” Electronic Letters, 2009, pp. 45, 219-221 discloses the production and function demonstration of a multimode waveguide produced in a volume of a resist material and/or a multi-core waveguide which connects a VCSEL (vertical cavity surface emitting laser) and a photodiode. With the integrated components to be connected, light is coupled in and coupled out via the surface of the substrate. The problem of connecting to a single-mode planar integrated waveguide does not arise with this method. The problem upon which the present invention is based thus cannot be solved with the above-described method. The structure described therein furthermore has a very low refractive index (estimated at 0.005) which does not allow reaching the high integration density required for photonic wire bonds.
Schmidt, V; Kuna L.; Satzinger, V; Houbertz, R.; Jakopic, G. & Leising G.; “Application of two-photon 3D lithography for the fabrication of embedded ORMOCER waveguides;” Porc. SPIE, Vol. 6476 discloses the production of waveguide structures with two-photon polymerization. The waveguides are based on a local increase in the refractive index which is induced by the radiation. The waveguides connect VCSELs with the associated photodiodes. The diameter of the waveguides is described as measuring “tens of microns” and it may be assumed that multimodal waveguides are used here as well which is confirmed by the intensity distribution shown in the publication. The described waveguides are therefore in principle not suitable for connecting single-mode integrated optical components. Accordingly; connecting of embedded waveguides to planar integrated lateral single-mode waveguide structures is not taken into consideration. The low refractive index difference furthermore results in extremely large structures (length of waveguide 2-12 cm), which gives reason to assume correspondingly large radii for the waveguide curvatures. The integration density necessary for photonic wire bonds cannot be achieved with this method. The specification furthermore discloses that the complete waveguide cross section is generated during a single writing passage. For this purpose, a telescope composed of cylindrical lenses is arranged in the beam path which allows a corresponding adaptation of the shape of the focusing region in the resist material. From this it can be assumed that the spatial resolution that can be achieved with the aforementioned arrangement is in the range of 10 μm, which prevents an efficient optical connection of the generated structures to lateral single-mode, planar integrated waveguides. The production method described in the publication furthermore contains a position detection of the optical components to be connected with the aid of a so-called “machine vision system.” A CCD camera is therefore used for the lateral position detection and is installed adjacent to the microscope objective. The accuracies which can be achieved with this arrangement are limited and, at best, should amount to a few micrometers. This is sufficient for multimodal connection waveguides. However, the connecting of single-mode connection waveguides to pre-positioned components is not possible with this system for lack of accuracy. In the axial direction, the machine-vision system only detects the position of the sample surface with the aid of a confocal arrangement. A three-dimensional position detection of components embedded in the resist material is not intended and, accordingly, this arrangement cannot be used to generate waveguide structures which are connected directly and with high precision to planar integrated waveguide-based components. The measured insertion loss for waveguides produced in this way amounts to 7.8 dB, wherein such high values cannot be tolerated when connecting nano-photonic systems.
Houbertz, R.; Satzinger, V; Schmid V; Leeb, W. & Langer, “Optoelectronic printed circuit board: 3D structures written by two-photon absorption; Organic 3D Photonics Materials and Devices II,” SPIE Int. Soc. Optical Engineering, 2008, Proceedings Vol. 7053, B530-B530 is closely connected to the above-discussed publication. It describes the production of waveguide structures for connecting pre-positioned components. A local increase of the refractive index, induced by a two-photon process, is used also in this case to define waveguide structures. The resulting structures, however, have extremely large cross sections and are therefore multimodal and not suitable for solving the object of the present invention. The waveguides furthermore have a low index contrast and correspondingly large curvature radii and therefore cannot meet the requirements for photonic wire bonds with respect to the integration density. Again, the lateral position of the components to be connected is detected with the aid of a camera installed adjacent to the microscope objective. The position detection in axial direction is limited to the detection of the upper edges of the VCSEL and photo diode chips which are fixated perpendicular to the surface of the component carrier. It can be assumed that the relative position accuracies which can be achieved with this arrangement are not sufficient for connecting single-mode planar integrated waveguides having cross sections of only a few micrometers. The generated waveguide structures furthermore cannot be connected directly to the surface of the chips—the waveguides end (start) at a distance of approximately 10 μm to the photodiode (the VCSEL). Technical reasons for this are not disclosed and it has be assumed that the shadowing effects caused by the vertically mounted chips play a role. The above-described arrangement cannot be used for producing photonic wire bonds which can be connected directly via corresponding connecting structures to planar integrated waveguides.
Houbertz, R; Wolter, H.; Dannberg, P; Serbin, J. & Uhlig, S.; “Advanced packaging materials for optical applications: bridging the gap between nm-size structures and large-area panel processing;” Art. No. 612605, Photonics Packaging and Integration VI, 2006, pp. 6126, 12605-12605 discusses inorganic-organic hybrid polymers (so-called ormoceres) with the associated structuring methods based on two-photon polymerization and their uses for the optoelectronics. Discussed as example, among other things, is the production of optical components with two-photon polymerization, wherein it is mentioned as an advantage that these components can be realized on substrates which already contain pre-structured components such as VCSELs or micro-lenses. However, the publication does not discuss the coupling of TPP structured waveguides with integrated optical waveguides. The waveguides described therein are multimodal and thus cannot be coupled without loss to single-mode planar integrated waveguides. The problem defined for the invention therefore cannot be solved with the methods described in this publication.